A stem cell is an undifferentiated cell from which specialized cells are subsequently derived. Examples of stem cells in the human body include pluripotent stem cells, embryonic stem cells, adult stem cells, fetal stem cells, and amniotic stem cells. Embryonic stem cells possess extensive self-renewal capacity and pluripotency with the potential to differentiate into cells of all three germ layers.
Totipotency refers to a cell that has the ability to differentiate into any cell in the body, including extraembryonic tissue. Pluripotency refers to a cell that has the potential to differentiate into cells of all three germ layers. Pluripotent cells however cannot form extraembryonic tissue, as a totipotent cell can. Multipotency refers to a cell that can differentiate into cells of limited lineage. For example, a hematopoietic stem cell can differentiate into several types of blood cells, but cannot differentiate into a brain cell.
The process by which a stem cell changes into a more specialized cell is referred to as differentiation. For example, some differentiated cells include endothelial cells, which are derived from endothelial stem cells.
The process by which a specialized cell reverts back to a higher degree of potency (i.e. to an earlier developmental stage) is referred to as dedifferentiation. In particular, cells in a cell culture can lose properties they originally had, such as protein expression or shape. It would be desirable to reduce the rate of dedifferentiation, or in other words to maintain the phenotype of differentiated cells in a cell culture.